A valve drive system is known in which, among a plurality of intake valves arranged at each cylinder, some valve(s) is of variable phase and the remaining valve(s) is of fixed phase (see Patent Document 1, for example).
In addition, a high expansion ratio internal combustion engine is known which includes: a variable compression ratio mechanism that varies a ratio between an internal volume of a cylinder at the time a piston is located at a top dead center (a combustion chamber volume) and an internal volume of the cylinder at the time the piston is located at a bottom dead center (a mechanical compression ratio); and a variable valve train that varies a ratio between the combustion chamber volume and an internal volume of the cylinder at the time an intake valve is closed (an effective compression ratio), wherein the expansion ratio to increased to a maximum extent by decreasing the effective compression ratio while increasing the mechanical compression ratio when the internal combustion engine is under relatively low-load (see Patent Document 2, for example).    Patent Document 1: Japanese Unexamined Patent Publication No. H03-088907    Patent Document 2: Japanese Unexamined Patent Publication No. 2007-303423